The wheel bearing apparatus to support a vehicle wheel is an apparatus that rotatably supports a wheel mounting wheel hub, via double row rolling bearings, including both driving wheels and a driven wheels. For structural reasons, a bearing of the inner ring rotation type is used for a driving wheel and both inner ring rotation and outer ring rotation types are used for a driven wheel. In general, the wheel bearing apparatus is classified into a so-called first-generation type where the wheel bearing comprises double row angular ball bearings fit between a knuckle and a wheel hub. In a second-generation type, the body mounting flange or the wheel mounting flange is directly formed on the outer circumference of the outer member. In a third generation type, one of inner raceway surfaces is directly formed on the outer circumference of the wheel hub. In a fourth generation type, an inner raceway surface is formed directly on the outer circumference of the outer joint member of the wheel hub and the constant velocity universal joint, respectively.
A wheel is rotatably supported relative to a suspension apparatus by a double row rolling bearing. A wheel bearing, including a double row taper roller bearing, is used for heavy loaded vehicles, such as off-road vehicles or trucks. In this wheel bearing, a hardened layer is formed by high frequency induction heating. The layer is on a range from an inner side end of a surface layer portion of the wheel hub to a root portion of a wheel mounting flange (see Japanese Laid-open Patent Publication No. 51064/1999).
Heavy duty vehicle wheel bearings require an increase in the rigidity of the wheel mounting flange of the wheel hub. Thus, the hardened layer is formed to satisfy this requirement. In addition, an inner ring fitting surface in the hardened layer of the wheel hub is required to improve its anti-creep properties.
In a third generation wheel bearing of the inner ring rotation type, the inner ring is fit onto the wheel hub and is axially secured to it by caulking the inner side end of the wheel hub. This type is a mainstream in wheel bearings for a driven wheel. FIG. 6 shows a representative example of this type of wheel bearing. This third generation type wheel bearing includes an inner member 51, an outer member 60, and double row balls 55, 55 contained between the inner and outer members 51, 60. The inner member 51 includes a wheel hub 52. An inner ring 53 is press fit onto the wheel hub 52 via a predetermined interface. In descriptions below, the term “outer side” defines a side that is positioned outside of the vehicle body (a left-hand side in drawings). The term “inner side” defines a side that is positioned inside of the vehicle body (a right-hand side in drawings) when the wheel bearing apparatus is mounted on the vehicle body.
The wheel hub 52 is integrally formed with a wheel mounting flange 54 at one end. Also, it includes an inner raceway surface 52a and a cylindrical portion 52b axially extending from the inner raceway surface 52a. The inner ring 53, formed with an inner raceway surface 53a on its outer circumference, is press fit onto the cylindrical portion 52b. It is axially secured relative to the wheel hub 52 by a caulked portion 52. The caulked portion is formed by plastically deforming, radially outwardly, the end of the cylindrical portion 52b. 
The outer member 60 is integrally formed with a body mounting flange 60b on its outer circumference. The body mounting flange 60b is adapted to be mounted on a body of a vehicle. The outer member 60 inner circumference has double row outer raceway surfaces 60a, 60a. A plurality of balls 55, 55 are freely rollably held by cages 56, 56 between the outer and inner raceway surfaces 60a, 52a; 60a, 53a. In addition, seals 57, 58 are mounted on both ends of the outer member 60. The seals prevent leakage of grease contained within the wheel bearing and ingress of rain water or dusts into the wheel bearing.
A heat treated layer 59 is formed in a surface layer portion. It is formed over a range from the cylindrical portion 52b of the wheel hub 52 to a root portion 54a of the wheel mounting flange 54. As shown in FIG. 7, the position “A” of the inner side end of the heat treated layer 59 is set within a range at a distance of 8 mm or less from the inner side end face (larger end face) of the inner ring 53 and of the outer side from an edge “B” of a chamfered portion 53c of the inner circumference of the inner ring 53. This makes it possible to axially secure the inner ring 53 by caulking. This improves the rigidity of the wheel mounting flange 54 and the anti-creep properties of the cylindrical portion 52b of the wheel hub 52. Also, it prevents deterioration of life of the wheel bearing due to deformations of the inner ring 53 and the inner raceway surface 53a caused by the caulking (see Japanese Laid-open Patent Publication No. 76346/2006).